The present invention relates generally to semiconductor device manufacturing, and, more particularly, to a method and apparatus for improved ellipsometric measurement of ultrathin films.
Ellipsometry is an optical technique that uses polarized light to probe the properties of a sample. One of the most common applications of ellipsometry is the analysis of thin films. Through the analysis of the state of polarization of the light that interacts with a sample, ellipsometry can yield certain information about the properties of such films. For example, depending on what is already known about the sample, the technique can probe a range of properties including the layer thickness, index of refraction, morphology, or chemical composition.
Generally, optical ellipsometry may be defined as the measurement of the state of polarized light waves. An ellipsometer measures the changes in the polarization state of light when it interacts with a sample. The most common ellipsometer configuration is a reflection ellipsometer, although transmission ellipsometers are also sometimes used. If linearly polarized light of a known orientation is reflected or transmitted at oblique incidence from a sample surface, then the resultant light becomes elliptically polarized. The shape and orientation of the ellipse depends on the angle of incidence, the direction of the polarization of the incident light, the wavelength of the incident light, and the Fresnel properties of the surface.
The polarization of the light is measured for use in determining certain characteristics of the sample. For example, in one conventional null ellipsometer, the polarization of the reflected light may be measured with a quarter-wave plate, followed by an analyzer. The orientation of the quarter-wave plate and the analyzer are varied until no light passes though the analyzer (i.e., a null is attained). Based on these orientations and the direction of polarization of the incident light, a description of the state of polarization of the light reflected from the surface may be calculated and the sample properties deduced.
Two characteristics of ellipsometry make its use particularly attractive in the field of semiconductor manufacturing. First, since ellipsometry is a nondestructive technique, it is suitable for in situ observation of a sample. Second, the technique is extremely sensitive in that small changes of a film may, in certain instances, be measured down to a sub-monolayer of atoms or molecules. Accordingly, ellipsometry has been widely used in areas such as physics, chemistry, materials science, biology, metallurgical engineering and biomedical engineering, to name a few. At the same time, however, advances in microelectronics fabrication are rapidly surpassing current capabilities in metrology. In order to enable the continued scaling of future generations of microelectronics, advances in specific metrology capabilities must also follow suit, such as the ability to measure the properties of ultra-thin films (e.g., thicknesses on the order of 20 angstroms or less) over sub-micron lateral dimensions.
Unfortunately, existing ellipsometry systems have difficulty in measuring and distinguishing between certain characteristics (e.g., index of refraction, thickness, etc.) of ultrathin films having varying optical properties. In the past, certain optical properties (such as material composition) have been assumed for thin films such as gate dielectrics where the dielectric material utilized was an oxide or nitride material, for example. However, with the use of more advanced ultrathin gate dielectrics, the traditional assumptions as to the composition of the dielectric material are no longer reliable for use in ellipsometric measurements. In particular, these ultrathin films do not produce enough of a phase shift on an incident beam to adequately distinguish between film thickness and film composition. Thus, a need exists for improving conventional ellipsometric techniques so as to be able to reliably obtain the desired measurements of advanced ultrathin films.